The present invention relates to apparatus and methods of producing concrete and to the component parts for such apparatus individually and in combination. More particularly it relates to apparatus and methods for mixing fine materials and water to form cementitious grouts and other such slurries: to apparatus and methods for cyclonically mixing aggregates and combining the aggregate mix with the grout to form concrete; and to aggregate weighing systems for delivery to such cyclonic aggregate blenders.
Modern concretes are required to be of a super performance standard and are composed of cementitious materials, water, chemical additives, fine and coarse aggregates mixed into a homogeneous mass which can be easily placed into a concrete structure. The cementitious materials and other fine materials included in concrete include such materials as, silica fume, pulverised fuel ash, and fine clay and mineral particles. These fine materials often agglomerate in clusters due to densification caused by packing (intentional or unintentional) and by the normal aggregation of fine particles.
These particles range in size from 0.02 micron to 1000 micron and aggregate into clusters having sizes of from 10 to 200 times the size of the individual particles. The clusters are caused to form, and are held together by, electrostatic or Van der Waals forces, mechanical adhesion, fusion etc. The aggregation of the particles prevents the intimate mixing of individual particles of different sizes together with the maximum packing density. This in turn is detrimental to the strength of the concrete structure formed from the concrete. The mixing of these materials varies from a simple stationary mixer to the specialised central mixing plants delivering mixed concrete to trucks for transportation to the construction site.
The mixer may be one of several types:
A) Dry Mixing: the most common process as there is no actual mixing plant, but the truck transportation bowl is designed as a mixing unit, while the concrete is being transported. This process is usually known as dry mixing as all the ingredients are loaded into the truck mixing bowl in a dry form together with the water and mixing takes place as the truck is transporting the materials to the site.
B) Central mixing: the equipment used usually consist of a specialised stationary mixing bowl and moving blades or paddles to rotate and mix the ingredients. The types and properties of concrete can vary widely and so do the types of equipment to mix the concrete.
As concrete mixtures have become more technical to achieve super properties the concrete ingredients and the mixing of those ingredients has become a highly specialised technology. The aim of the mixing process is to separate the individual cement particles and to surround these individual particles with individual particles of additives, such as silica fume. The cement and additive mixture should then surround the aggregate and fill the voids between individual pieces of aggregate. As the cement and additive particles are typically of very small dimensions considerable difficulty has been experienced in achieving the ideal mixing outcomes.
Conventional mixing processes are based on the philosophy of mixing the cements or cementitious materials with the aggregate and water to produce a consistent homogeneous mass of workable concrete. The mixing is achieved by the rotation of the blades or paddles moving the larger aggregates to inter-grind the cementitious materials.
The existing technology largely relies on mechanical mixing where the aggregates themselves can provide a part of the process of inter-grinding of the cementitious materials into the aggregates. The prior art mixing processes for the production of cementitious materials have the following disadvantages:
1. It is usually necessary to extend the grout mixing process over a long period of time to ensure that it is mixed due to material coagulation and lumps.
2. The machines described above are not capable of mixing stiff viscous materials.
3. Grout materials must be pumped some distance to be combined with the aggregates and there is usually considerable difficulty in emptying the discharge lines of the exact weighed materials originally present in the mixed proportions as there are no further materials to clear the lines, except by gravity flow which is very slow.
4. As the aggregates have been transported usually by conveyor into an aggregate holding bin these materials are usually segregated and on entering the mixing truck and together with the grouts require substantial mixing times (4 to 10 minutes).
An alternative process is to mix all the cementitious materials together with water and the chemical additives into a liquid grout which is then combined with the aggregates which, after mixing produces a homogeneous mass of workable concrete.
The present invention builds upon the alternative mixing process for concrete and addresses to problems outlined above in respect of forming the grout that is to be mixed with the aggregate from finely powdered materials. The invention provides an apparatus and a method for forming the liquid grout; apparatus and a method for cyclonically mixing aggregates to reduce segregation; to aggregate weighing systems; and to apparatus and a method combining the foregoing for mixing the liquid grout with the aggregate material to form concrete.
In a first aspect the present invention relates to a method of forming a slurry of particles of fine materials in a liquid, wherein the mixing and dividing of agglomerations of the particles of fine materials is carried out by subjecting a slurry of the agglomerations of particles to the action of one or more rotating discs shaped and arranged to create within the slurry alternating areas of high and low pressure, bubbles being formed in the low pressure zones and being compressed in the high pressure zones to implode with great force thereby breaking up the agglomerations of particles and at the same time separating and dispersing the individual particles of fine materials.
In a second aspect the present invention relates to an apparatus for forming a slurry of particles of fine material in a liquid, the apparatus including a housing adapted to receive and hold a slurry of particles of fine materials in a liquid which slurry contains agglomerations of the particles, and, located within the housing, one or more rotatable blades shaped and arranged to create within the slurry alternating areas of high and low pressure such that bubbles are formed in the low pressure zones and are compressed in the high pressure zones to implode with great force thereby breaking up the agglomerations of particles and at the same time separating and dispersing the individual particles of fine materials.
The method and apparatus according to the first and second aspects of the present invention could be used for the formation of slurries of any suitable fine material in any suitable liquid. The invention is however of particular applicability in the preparation of slurries of cementitious materials, such slurries being typically referred to as grouts. Such grouts may be mixed with aggregates to form concrete or may be used for other purposes.
In a third aspect the present invention relates to a method for the mixing of aggregate including conveying aggregate materials into a substantially conical hopper simultaneously along at least two conveyors disposed substantially tangentially to the hopper such that the aggregate is caused to enter the hopper along a substantially helical path with the aggregate from the said two conveyors travelling in opposite directions around the hopper.
In a fourth aspect the present invention relates to apparatus for the mixing of aggregate including a substantially conical hopper and at least two conveyors disposed substantially tangentially to the hopper, the conveyors being adapted to convey aggregate to the hopper such that the aggregate is caused to enter the hopper along a substantially helical path with the aggregate from the said two conveyors travelling in opposite directions around the hopper.
The method and apparatus according to the present invention may be used for mixing aggregate for a variety of purposes however they are of particular applicability in the mixing of aggregate of different sizes for inclusion in concrete.
In a fifth aspect the present invention relates to a method of mixing of grouts of a cementitious materials and the combining of the grouts with aggregate materials to be contained in concrete, the cementitious materials together with water and chemical additives being mixed in a grout forming apparatus according to the second aspect of the present invention, the aggregate being fed into an aggregate cyclone blender according to the fourth aspect of the present invention and thereafter the grout from the grout forming apparatus is delivered to the aggregate cyclone blender and mixed with the aggregate, the mixture being discharged from the aggregate cyclone blender into a secondary mixing apparatus.
In a sixth aspect the present invention relates to concrete producing apparatus for mixing grouts of cementitious materials and combining such grouts with aggregate materials to be contained in concrete, the apparatus including a grout forming apparatus according to the second aspect of the present invention disposed such that it can discharge into an aggregate cyclone blender according to the present invention.
In a seventh aspect the present invention relates to aggregate weighing means including a plurality of aggregate storage bins disposed above a conveyor, means to selectively release aggregate from one or more of the bins onto the conveyor, and means to vary the speed of the conveyor in accordance with the bin or bins from which the aggregate is being released.